1. Field of the Invention
The present invention relates generally to a transmission apparatus and method in a digital broadcasting system. More particularly, the present invention relates to an apparatus and method for reducing a Peak to Average Power Ratio (PAPR) of a preamble signal in a digital broadcasting system.
2. Description of the Related Art
Digital broadcasting systems are broadcasting systems that use digital transmission technologies, such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB) and Digital Multimedia Broadcasting (DMB).
Among them, the DVB system, which is a European digital broadcasting technology, is a transmission standard that supports existing digital broadcast services for fixed terminals as well as digital multimedia services for mobile and portable terminals.
In the DVB system, it is possible to multiplex Moving Picture Experts Group 2 Transport Stream (MPEG 2 TS)-based broadcast data and simultaneously transmit Internet Protocol (IP)-based data streams. Further, in the DVB system, several services can be multiplexed to one IP stream and transmitted. After receiving data of the transmitted IP stream, a user terminal can demultiplex the received data back into individual services, demodulate the services, and display the demodulated services on a screen of the user terminal. In this case, the user terminal requires information about various types of services provided in the DVB system, the details contained in each of the services, etc.
The DVB system uses an Orthogonal Frequency Division Multiplexing (OFDM) transmission scheme. Although the OFDM transmission scheme is similar to the conventional Frequency Division Multiplexing (FDM) scheme, the OFDM scheme is distinguishable from conventional transmission schemes in that OFDM can achieve optimal transmission efficiency during high-speed data transmission by maintaining orthogonality between multiple sub-carriers. Also, the advantages of high frequency use efficiency and robustness against multi-path fading contribute significantly to the optimal transmission efficiency.
In addition, the OFDM transmission scheme, as it overlaps frequency spectra, is efficiently uses frequencies, is robust against frequency selective fading, can reduce influence of Inter-Symbol Interference (ISI) by using a guard interval, enables simple designs of hardware equalizers, and is robust against impulsive noises. Thus, the OFDM transmission scheme is widely used for communication systems.
Despite the advantages of the OFDM transmission scheme, the multi-carrier modulation of the OFDM transmission scheme causes a high Peak to Average Power Ratio (PAPR). Since the OFDM transmission scheme transmits data using multiple carriers, a final OFDM signal suffers a significant change in amplitude as a level of its amplitude is determined by a sum of amplitude levels of the respective carriers. When the carriers are equal in phase, the OFDM signal will have a very high amplitude. This high-PAPR signal may deviate from a linear operation range of a high-power linear amplifier, and a signal that has passed through the high-power linear amplifier may often suffer from distortion, causing a reduction in system performance.
Various plans to solve the high-PAPR problem occurring in the OFDM system have been proposed, and the plans include several PAPR reduction techniques, such as clipping, coding, SeLected Mapping (SLM), Partial Transmit Sequence (PTS), and Tone Injection (TI).
A Tone Reservation (TR) scheme, one of the PAPR reduction techniques, reserves some tones in sub-carriers, and the reserved tones are used to reduce PAPR instead of transmitting data. A receiver bypasses the reserved tones that do not carry information signals, and restores information signals only in data tones other than the reserved tones, thus contributing to a simplified structure of the receiver. A gradient algorithm is a typical one of the methods that reduce PAPR using reserved tones in the TR scheme. The gradient algorithm is defined by applying a method similar to the clipping technique to the TR scheme. The gradient algorithm is used to create a signal (or a kernel) having an impulse characteristic using reserved tones that carry no information signal, and to clip off an output signal of an Inverse Fast Fourier Transform (IFFT) unit. When the signal having an impulse characteristic is added to the output signal of the IFFT unit, distortion of data occurs only in the reserved tones, and data in other frequency regions is not distorted, i.e., the TR scheme is different from the clipping technique in that the noise caused by clipping has an effect only on some reserved sub-carriers without affecting all sub-carriers. The gradient algorithm optimizes impulse waveforms so that a peak of an IFFT output signal is reduced in the time domain. A signal, a PAPR of which was reduced by adding a sum of the impulse waveforms optimized by the gradient algorithm to the output signal of the IFFT unit, is transmitted to a receiver. The receiver only needs to receive data on the remaining sub-carriers since the receiver is notified of the locations of the reserved tones in advance.
FIG. 1 illustrates a structure of a transmitter to which a general TR scheme is applied.
Referring to FIG. 1, an input signal X 105 having N-L points (where N denotes a size of IFFT) and an L-reserved tone signal C 110 (a signal consisting of L reserved tones) are input to a tone reservation unit 120, and the tone reservation unit 120 reserves L reserved tones in sub-carrier locations previously agreed between a transmitter and a receiver. The L reserved tones carry no data and have zeros (0s) inserted therein. When a sum of the parallel data X and the L reserved tones is input to an N-point IFFT unit 130, the input sum undergoes IFFT computation in the N-point IFFT unit 130, and then a time-domain output signal x is generated by a parallel-to-serial (P/S) conversion unit 140. Next, a gradient unit 150 creates a signal c by optimizing impulse waveforms according to the gradient algorithm so that a peak of the IFFT output signal is reduced, and adds the created signal c to the output signal x that has passed through the IFFT unit 130 and the P/S conversion unit 140. The output signal of the gradient unit 150 is transmitted to the receiver. The gradient unit 150 calculates the signal c that is added to the output signal x so that PAPR of the output signal x is reduced, using impulse waveforms read from a memory 160. For reference, uppercases X and C in FIG. 1 represent frequency-domain signals being input to the IFFT unit 130, while lowercases x and c represent time-domain signals output from the IFFT unit 130.
The signal c that is added to the output signal x to reduce PAPR in L reserved tones, is determined as follows. L sub-carriers are reserved in advance and used to determine a code C for calculation of the signal c, and locations of the L sub-carriers are fixed by the tone reservation unit 120 during initial transmission, and remain unchanged during data transmission. The code C represents the reserved tone signal, and Ck represents sub-carrier locations of reserved tones as defined in Equation (1) below.
                              C          k                =                  {                                                                                          C                    k                                    ,                                                                              k                  ∈                                      {                                                                  i                        1                                            ,                                              i                        2                                            ,                      …                      ⁢                                                                                          ,                                              i                        L                                                              }                                                                                                                        0                  ,                                                                              k                  ∉                                      {                                                                  i                        1                                            ,                                              i                        2                                            ,                      …                      ⁢                                                                                          ,                                              i                        L                                                              }                                                                                                          (        1        )            where k denotes sub-carrier indexes of reserved tones in the tone reservation unit 110. The input signal X 105 is reserved in sub-carriers aside from the reserved tone signal C 110 as shown in Equation (2).
                              X          k                =                  {                                                                                          X                    k                                    ,                                                                              k                  ∉                                      {                                                                  i                        1                                            ,                                              i                        2                                            ,                      …                      ⁢                                                                                          ,                                              i                        L                                                              }                                                                                                                        0                  ,                                                                              k                  ∈                                      {                                                                  i                        1                                            ,                                              i                        2                                            ,                      …                      ⁢                                                                                          ,                                              i                        L                                                              }                                                                                                          (        2        )            where Xk denotes sub-carrier locations of data tones.
PAPR minimization is achieved by optimizing amplitudes of the L sub-carriers. {tilde over (C)} for PAPR minimization is optimized by Equation (3) below such that PAPR of an output signal x is reduced. Here, {tilde over (C)} corresponds to the signal c.
                              C          ~                =                  Arg          ⁢                                          ⁢                                    min                              C                ~                                      ⁢                          (                                                max                                      n                    =                                          0                      -                      N                      -                      1                                                                      ⁢                                                                                              x                      u                                        +                                          c                      n                                                                                                    )                                                          (        3        )            where Cn is a value of an n-th element in a time-domain vector c determined by IFFT-processing a vector C. Computation of Equation (3) is performed to find an optimized signal for the signal c. Although complex linear computation is conducted to solve Equation (3), the gradient algorithm is performed in actual implementations, which can achieve the similar performance through simple computations.
The signal c is optimized to remove a peak-to-peak value of a vector x (i.e., an output signal). If xclip is assumed to be a vector where an output signal x is clipped to a certain level A, then Equation (4) is derived.x−xclip=Σiβiδ[n−mi]  (4)where βi denotes a clipping value, mi denotes a location where the vector is clipped, and δ denotes an impulse function.
If the signal c is defined as Equation (5), Equation (6) can be derived and a peak-to-peak value of a transmission symbol can be reduced.c=−Σiβiδ[n−mi]  (5)x+c=xclip  (6)
Therefore, the signal c added to the output signal x can be construed as a sum of delayed and scaled impulse functions. However, in a frequency domain, Ĉ=FFT(c) has a non-zero value at most frequencies, and distorts values of data symbols in locations other than the reserved L locations. Thus, there is a need to use, for clipping, waveforms having characteristics of an impulse function, which are affected only in the L reserved locations but not affected in other locations in the frequency domain.
Waveforms having impulse characteristics are designed as follows.
Assume that 1L represents a vector having a value of 1 in L reserved locations and a value of 0 in other locations, and p is defined as Equation (7).
                    p        =                              p            ⁡                          [              n              ]                                =                                    [                                                p                  0                                ⁢                                  p                  1                                ⁢                                                                  ⁢                …                ⁢                                                                  ⁢                                  p                                      N                    -                    1                                                              ]                        =                                                            N                                1                            ⁢                              IFFT                ⁡                                  (                                      1                    L                                    )                                                                                        (        7        )            
In Equation (7), p0=1, and p1 . . . pN-1 have significantly small values compared to p0. Assuming that p[((n−mi))N] indicates a value determined by cyclic-shifting p by mi, even though Discrete Fourier Transform (DFT) is performed thereon, the resulting value changes only in phase and has a value of 0 in locations other than the L reserved locations in the frequency domain.
As described above, the waveforms having impulse characteristics are designed such that amplitudes of the remaining p1 . . . pN-1 except for p0 are low, in order for the waveforms to become similar to ideal impulse waveforms. As amplitudes of p1 . . . pN-1 are lower, a change in amplitudes of other signals except for p0 is less significant during clipping. If the design is made such that p1 . . . pN-1 have high amplitudes, peaks of other signals may increase again in the clipping process, thereby causing a reduction in PAPR reduction performance.
FIG. 2 illustrates a frame structure for a physical layer of a general DVB system using OFDM.
A frame structure 201 of FIG. 2 can be roughly divided into preamble parts 202 and 203, and a payload part 204. The preamble parts 202 and 203 carry signaling information of the frame, and the payload part 204 is used to transmit data.
The P1 preamble 202 is used at a receiver to scan an initial signal of the frame. Further, the P1 preamble 202 is used to detect a frequency offset and tune the center frequency. Next, the P2 preamble 203 is used to provide Layer 1 (L1) signaling of the DVB system. The L1 signaling includes such information as transmission types, transmission parameters, etc. of the DVB system. Finally, the payload 204 carries service data provided in the DVB system.
In the communication system that uses the OFDM transmission scheme and transmits the frame including preambles, it is important to reduce PAPR in order to improve the system performance, and the high-PAPR problem occurs not only in the payload part in which data is transmitted, but also in the preamble parts in which signaling information is transmitted, in the physical layer frame. Therefore, there is a need for reducing PAPR, including reducing PAPR in the preamble parts of a frame.